1. Field of the Invention
The present invention generally relates to a mobile communication system and a base station, and more particularly, to a mobile communication system and a base station in which communications are carried out by mobile station equipment for a personal handy phone system in Japan, etc.
Mobile telephones and portable telephones are now widely used, and further, a personal handy phone system (PHS) service has been provided for commercial use in Japan since 1995. The personal handy phone system has a feature that configurations of mobile station and base station equipment are simpler as compared to the conventional automobile and portable telephone systems. Transmission power of portable station equipment in the personal handy phone system is around 10 mW. A usage frequency band for communication between the base station and the portable station is a 1.9-GHz band. A speech coding method used in the system is ADPCM (Adaptive Differential Pulse-Code Modulation) of 32 kbps. And a TDMA/TDD (Time-Division Multiple Access/Time-Division Demand) method is applied to an access method of the system. In such a mobile communication system, an effective operation of the base station is desired.
2. Description of the Prior Art
FIG. 1 shows a configuration of the personal handy phone system (PHS) in Japan. In FIG. 1, numerals 71-1 to 71-4 indicate base stations (CS), a numeral 72 indicates a portable station which is commonly referred to as a mobile station, a numeral 73 indicates a private branch exchange (PBX), a numeral 74 indicates a public network, and symbols S1 to S2 indicate service areas for the control stations.
As mentioned before, the mobile station 72 communicates with the base stations 71-1 to 71-4, with the transmission power of 10 mW, at the frequency band of 1.9 GHz, by the ADPCM of 32 kbps and TDMA/TDD access method. In the TDMA/TDD method used in the personal handy phone system in Japan, 1 frame of 5 ms comprises 4 timeslots for down link channels from the base station to the mobile station and 4 timeslots for up link channels from the mobile station to the base station, where 1 timeslot is 625 .mu.s. And by transmitting successive frames to each other, the communication between the mobile station and the base station is carried out.
In FIG. 1, the base stations 71-1, 71-2 are connected to the public network 74 through PBX 73, and the base stations 71-3, 71-4 are directly connected to the public network 74. For connections between the base stations 71-1, 71-2 and PBX 73, between PBX 73 and the public network 74, and between the base stations 71-3, 71-4 and the public network 74, ISDN lines are commonly used. A B channel of the ISDN line has a transmission rate of 64 kbps. Therefore, each of the base stations 71-1 to 71-4 needs a conversion function between the 64-kbps B channel of ISDN and a 32-kbps speech channel of ADPCM.
When PBX 73 and the base stations 71-1, 71-2 are provided in a building, the service areas S1, S2 may be formed on each floor by providing the base stations 71-1, 71-2 for each floor. The mobile station 72 in the service areas S1, S2 communicates with a fixed telephone connected to the public network 74 through PBX 73 and the public network 74. The mobile station in the service areas S1, S2 can also communicate with the other mobile stations 72 in the service areas S3, S4 through PBX 73 and the public network 74. The mobile stations 72 in the service area S1 can communicate with the mobile stations 72 in the service areas S2 through PBX 73. And, the mobile stations 72 in the same service area S1, S2 communicate with each other through PBX 73. In the same way, the mobile stations 72 in the service areas S3, S4 communicate with other phones through the public network 74. To manage distributed base stations, a generalizing base station may be provided.
However, there is a problem in the conventional personal handy phone system as described in the following.
In the conventional system using PBX 73, in an office, etc., when the mobile stations 72 in the same service area S1, S2 communicate with each other, the mobile stations 72 are connected to each other by the exchange connection control in the PBX 73. In the conventional system without PBX 73, in the same way, when the mobile stations 72 in the same service area S3, S4 communicate with each other, the mobile stations 72 can be connected to each other through the public network 74. For the case that the mobile stations in the same service area communicate with each other, a method of directly communicating with each other is proposed. However, in this method, it is necessary to set conditions for the direct communication in both mobile stations. Therefore, when the mobile station is not set with the conditions for the direct communication, the mobile station cannot directly communicate with an other mobile station in the same service area because the non-set mobile station cannot be called.
As mentioned above, for the communication between the mobile stations in the same service area, the exchange in the PBX or the switch in the public network are used to perform the communication. Therefore, a processing load in the exchange and the switch increases. More specifically, in the office having no PBX 73, the communication between the mobile stations in the same area 71-3, or 71-4 is carried out through the public network 74, just like the automobile and portable telephone systems. Therefore, a traffic in the public network 74 increases and a whole control operation in the public network 74 is complicated. Further, in this case, since the communication is connected through the switch in the public network 74, a call's fee is charged for the communication based on a communication time.